Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2004
Publication Date: 8/5/2004
Citation: Northup, B.K., Dias, C.D., Brown, J.R., Skelly, W.C. 2004. Micro-patch and community scale spatial distribution of herbaceous cover in a grazed north Australian grassland. Journal of Arid Environments. 60(3):510-530. Interpretive Summary: Grass plants in the dry tropics of Australia are irregularly distributed, and their pattern of distribution can change with scale of measurement, grazing and climate. The pattern of grass cover can be distributed at fine scales, and is important to the function and productivity of Australian landscapes. To understand how grass communities function in response to grazing, the distribution of plants, and how grazing affects this distribution over time must be described. A study was conducted in 1997 to examine how the projected cover of grass plant canopies were arranged within three small (3.2 to 4.2 acre) experimental pastures, in different condition classes due to levels of grazing pressure applied in 1993 through 1997. Information was collected within pastures from both the plant community (entire pasture) and micro-patch (11.2 by 13.8 foot) scales, and analyzed to determine how cover was distributed at both scales, and to build maps of the distribution of cover. Grass cover was distributed irregularly within the three pastures at both the community and micro-patch scales, and changed with level of grazing. In the more lightly grazed pastures, high cover areas were present as bands running across the slopes, with the bands separated by areas with low, or no cover. On the most heavily grazed pastures, high cover areas were present as large patches with no organization related to slope. These patches were made up of Indian couch grass, an invading exotic grass, as compared to the bands of native bunch grasses on the other pastures. Describing how grass cover is distributed in this Australian grassland, and its response to management, requires that samples be collected from several different scale sizes to ensure the information has enough detail to pick up changes.
Technical Abstract: Vegetative characteristics of dry tropical Australian plant communities are highly variable at fine scales (<100m), and often at resolutions of <1.0 m. Such fine-grain patterning must be quantified to understand landscape function and effects of disturbance. In July-August 1997, spatial distributions of projected herbaceous cover were examined within three experimental paddocks established in dry eucalypt woodlands of northeast Australia, and in different condition (stable, degrading, degraded), due to grazing regimes applied in 1993-1997. Community-scale (1.3-1.7 ha) data (n=215/paddock) were obtained from spatially referenced digital images (3.4 * 4.2 m). Micro-patch (one per paddock) scale (3.0 * 4.2 m) data were collected by BOTANAL estimation techniques within grids of 0.3 *0.3 m (n=140) and 0.1 * 0.1 m cells (n=162). Data from each scale within each paddock were analyzed by geostatistical procedures to describe spatial distribution of cover, and interpolate distribution maps. High cover zone were distributed in bands across slopes and separated by areas of low cover on the stable and degrading communities (530 to 1800m2 and 380 to 800m2, respectively), and large patches (150 to 750 m2) without organization related to topography on the degraded community. Cover was more regularly distributed across micro-patches, with few abnormalities related to slope. High cover zones decreased as condition changed from stable (0.3-0.6m2) to degrading (<0.1-0.3 m2), then increased on the degraded (0.4-0.7m2) patch due to an invasive stoloniferous grass. The distribution of herbaceous cover in this north Australian plant community, and response to management, can only be definded by multiple scale sampling.